A high depth of field is desirable in various applications in which, in particular, three-dimensional objects are recorded by means of an image recording apparatus, for example in order to be able to image a desired region or the entire recorded object in focus in a two-dimensional image. Here, the depth of field is generally understood to mean a region in which an object is imaged sufficiently in focus, for example in accordance with a resolution of a recording medium such as an image sensor.
The numerical aperture can be reduced in conventional systems for the purposes of increasing the depth of field. However, this leads to a reduction in the resolution and/or in the available light at the same time.
The use of specific pupils and field filters provides a further option for increasing the depth of field of an apparatus for recording images. Examples of this are described in Edward R. Dowski, Jr., and W. Thomas Cathey, “Extended Depth of field through wave-front coding”, Applied Optics, Vol. 34, No. 11, 1859-1866, 1995. As a rule, such approaches are disadvantageous in that they significantly reduce the contrast at higher spatial frequencies, possibly leading to a loss of resolution on account of noise. Furthermore, artifacts may be formed during digital post-processing that is required in this case. Moreover, a specific optics design is necessary, in which the pupil plane/intermediate image plane is accessible and optically corrected, leading to increased costs of the apparatus.
A third conventional option for producing an image with an increased depth of field lies in a digital calculation of such an image from a so-called z-stack measurement. This approach is also referred to as “focus stacking”. In this approach, a plurality of images are recorded with different foci (for example by displacing the sample or by a different modification of the apparatus, for example by changing the focusing of an optical system). Then, the respective sharpest region of each image is transferred into the overall image by means of suitable algorithms. Here, it is possible to produce an image which has a virtually arbitrary depth of field, without this yielding a loss of resolution. A disadvantage of this approach is that the aforementioned z-stack, also referred to as defocus stack, needs to be produced by mechanical displacement of a component of the apparatus (for example displacing lenses for changing the focus and/or displacing the sample), which may take a relatively long period of time, may be comparatively high cost and/or may introduce unwanted movements of the sample when displacing the sample.
It is therefore an object to provide methods and apparatuses by means of which the above-described disadvantages can be at least partly overcome or reduced and by means of which it is possible to record images with an increased depth of field compared to conventional single image recording.